Final 'X-51A Waverider' hypersonic plane will try for Mach 6 next year

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And then there were none. The fourth and final X-51A Waverider hypersonic plane will be tested next spring or summer, the U.S. military announced Wednesday.

The third unmanned, experimental plane failed to fly last August after being dropped from a B-52 bomber off the coast of California near Point Mugu.

Prior to plummeting into the Pacific Ocean, the one-time-use, Boeing-built bullet was supposed to shoot through the sky for five minutes in an attempt to reach Mach 6, or 3,600 mph, or six-times the speed of sound.

Instead, a fault in the control fin caused the craft to lose balance and crash off the coast before activating its supersonic combustion ramjet (scramjet) engine. An investigation has ruled out software issues or power failures as the cause.

The AP reports that while the exact reason for the failure has not been identified, signs point to a "random vibration issue," according to program manager Charlie Brink at the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio.

None of the test flights have succeeded in reaching Mach 6. The first craft achieved Mach 5 for about 140 seconds in 2010. Test two ended prematurely in 2011 with the craft trying to restart its engine. And you already know the fate of flight three.

Other, similar hypersonic projects are also known to be under way. In November 2011, the Pentagon tested an "advanced hypersonic weapon," the Associated Press reported, and in August 2011, a glider dubbed HTV-2 peeled apart during a trial flight.

Hypersonic technology could allow for an Air Force strike to be carried out anywhere in the world in less than 60 minutes.